It has previously been proposed to control the output voltage of automotive type generators, typically alternators connected to rectifiers by using a control circuit or control loop in which the command voltage is the d-c output voltage of the generator, that is, typically the rectified voltage derived from the generator. Alternatively, the voltage on a battery of the vehicle can be used. Still, various types of rectifier systems use separate diodes to rectify current to the exciter field of the alternator, and the output voltage at the rectifying diodes for the exciter field may likewise be used as a command, or control voltage for the voltage regulator. There is a difference in the voltage levels of these three command or control voltages: some voltage drop arises in the power connection between the generator and the battery, voltage drop due to the diode voltages of the rectifier diodes; and voltage drop due to the diode voltages of the exciter rectifier diodes. The voltage drop on the power connection, and the voltage drop across the diodes of the rectifiers is dependent on the loading of the network.
Various types of control circuits have been proposed for different types of vehicles and for different uses of the resulting generator charging and voltage control system. When the command or control voltage is the battery voltage, the type of control is usually referred to as B+ control; controlling the voltage based on the d-c output voltage of the alternator, that is, the d-c voltage supplied by the power generator is referred to as B+Gen control. If voltage control is effected by sensing the voltage at the field exciter diodes, the control is usually referred to as D+ control.
B+ control requires a separate connection from the battery to the voltage regulator to supply the command or control voltage actually at the battery as a command value to the voltage regulator. The battery and voltage regulator are frequently located physically at separate positions and, in motor vehicles, may be spaced a good distance from each other. Thus, then, requires a control line of substantial length. This control line is subject to damage, for example failure, or break therein. Upon interruption of the control line, battery voltage will no longer be applied to the voltage controller; rather, the then applied level will be zero. Although the battery may be fully charged, the voltage regulator would react as if the alternator battery had zero voltage and, therefore, the alternator is commanded to supply more and more power at higher and higher voltages. This may lead to damage and possibly destruction of the alternator and/or the battery and/or the voltage regulator. Similar erroneous control may occur if the power connection between the alternator and the battery might break. This, also, opens the control loop and the generator is controlled to supply output voltages at a higher level which, however, it can no longer supply to the battery. If this condition persists for any appreciable period of time, both the alternator as well as the voltage regulator may be damaged or destroyed.